When an engine runs in a slow engine speed, the moving speed of a piston is slow, which does not likely to cause a turbulence of an air-fuel mixture in a combustion chamber. This slows the flame propagation speed of the air-fuel mixture ignited by a spark plug. In the case of a high-load condition, the output generated by an engine becomes larger than the case of a low-load condition. In this case, the amount of intake air introduced into the combustion chamber becomes large. The larger the amount of intake air is, the more the spontaneous ignition is likely to occur due to the compression by the piston. In such a case, the air-fuel mixture is spontaneously ignited before the flame of the air-fuel mixture ignited by the spark plug propagates.
Patent Document 1 discloses a technology focusing on such a problem.
In the technology disclosed in Patent Document 1, whether or not a state that causes an abnormal combustion occurs based on an intake pressure, an engine temperature, an engine speed, and an air-fuel ratio. When determining that the abnormal combustion will occur, the closing angle of an intake valve is retarded to decrease the charging efficiency to the combustion chamber, thereby suppressing an abnormal combustion.